The present invention relates to a control circuit for controlling the operation of a brushless DC motor to produce a constant level of output torque.
Due to the improvements which have been made in the characteristics of DC motors, these are being increasingly widely utilized as servomotors in a wide variety of equipment, in various fields of applications. In particular, this is true of brushless DC motors which employ non-contact magnetic field sensing means and electronic switching circuits to perform switching of armature current, rather than the conventional commutator. More specifically, such a DC brushless motor generally consists of a plurality of armature windings, a rotor including a permanently magnetized field magnet having a plurality of magnetic poles, such a rotor being referred to hereinafter as a magnetorotor, magnetic field sensors for sensing the current positions of the magnetic poles of the magnetorotor with respect to the stator windings, and an armature winding current supply circuit, i.e an electronic circuits for performing appropriate switching control of the armature winding currents in response to signals from the magnetic field sensors. for controlling the motor torque.
However the torque which is produced by a brushless DC motor is proportional to the product of the level of current flowing in the armature windings and the amount of magnetic flux of the field magnets of the magnetorotor that is linked to the armature windings. If the waveforms of the currents which flow in the armature windings are not accurately related to the distribution of magnetic flux produced by the field magnets of the magnetorotor such that this product of magnetic flux and current is constant, then periodic variations (pulsations) of the torque produced by the motor will occur, i.e. torque ripple will be generated. Due to this, the motor will not rotate in a precisely stable manner, and wow, flutter, jitter and vibration will result.
FIG. 1 is a torque generation block diagram of an example of a prior art current feedback 4-input 2-phase brushless DC motor. In FIG. 1, Vt represents a torque control voltage whose level can be adjusted to produce a required level of output torque, I denotes the motor drive current, T denotes the generated torque, and Vi denotes a current sensing voltage, while A denotes an adder. A differential voltage (Vt-Vi) is produced from the adder A. Block G denotes an amplifier which amplifies the differential voltage (Vt-Vi) from the adder A and converts this into a motor drive current I. .phi. denotes the process whereby the motor converts the magnetic flux .phi. of the motor field and the motor drive current I into a level of torque T. Block R denotes a voltage detector which converts the motor drive current I into a current sensing voltage Vi. Block A' denotes an adder, and block T' denotes torque ripple that is generated by the motor. This torque ripple from block T' is added to the torque from the motor block .phi. in adder A', to thereby produce the output torque T. FIG. 2 is a circuit diagram of an example of a prior art type of current feedback 4-input 2-phase brushless DC motor, whereby torque is generated by the process illustrated in FIG. 1. FIG. 3 is a cross-sectional view in elevation of a brushless DC motor, whose body is generally shaped as a flat disc, and in which an annular magnet 1 constitutes the field magnet of a magnetorotor. FIG. 4 is a plan illustration to show the positional relationships between the armature coils and the magnetorotor poles of the brushless DC motor of FIG. 3, showing the positional relationships between the field magnet 1 (annular magnet 1), armature windings L1 to L4, and position-sensing Hall generators H1 and H2.
The field magnet 1 in FIG. 3 and FIG. 4 is permanently magnetized with magnetic fields which are generally aligned with the direction of the rotor shaft 3 of the magnetorotor (as viewed in the drawings, the field magnet 1 is magnetized such that there are six poles). The field magnet 1 is fixedly attached to a yolk 4, which is formed of a highly magnetically permeable material. The yoke 4 is fixedly attached to the rotor shaft 3. In addition, a magnet 5 for generating a magnetic field for a speed sensing generator (frequency generator) is also attached to the yoke 4. The field generating magnet 5 of the speed sensing generator is permanently magnetized in a fixed magnet pattern, and as the motor rotor rotates, a speed sensing signal is generated, as an AC signal whose frequency varies in accordance with the speed of rotation of the magnetorotor, by a magnetic sensor mounted on a baseplate 7, which is mounted on the stator base 6 of the motor. The armature windings L1 to L4 are fixedly mounted on the stator base 6, as also are two Hall generators H1 and H2, and a bearing 8 for the magnetorotor shaft 3. The bearing 8 is formed of a bearing holder 8a, a radial bearing 8b, and a thrust bearing 8c.
As shown in FIG. 4 each of the armature windings L1 to L4 includes two radially extending portions, a representative one of which is indicated by the hatched-line portion L.sub.T. In the motor example shown, the field magnet has six poles, indicated as N, S,. . ., so that the angular pitch P between successively adjacent poles is 360.degree./6, i.e. 60.degree., and the effective torque-generating portions L.sub.T of the armature windings are positioned at equidistant angular intervals of P/2, i.e. 30.degree..
In FIG. 2, the portion of the circuit which is associated with transistor X1 corresponds to the portion indicated as the adder A in the torque generation block diagram of FIG. 1. Resistor R1 corresponds to the voltage sensing section denoted by R in FIG. 1, which serves to convert the motor drive current I into a current-sensing voltage Vi. The remaining portions of the circuit of FIG. 2, other than the armature windings L1 to L4 and the blocks A and R, correspond to blocks G, .phi. and T' and A' in the torque block diagram of FIG. 1.
The Hall generators H1 and H2 are disposed mutually separated by an electrical angle of 90.degree., and produce output signals to indicate the current position of the magnetorotor with respect to the armature windings, by sensing the magnetic flux from the poles of field magnet 1 of the magnetorotor. The resistors R4, R5 and R6, R7 respectively determine a suitable level of DC bias current for the Hall generators H1 and H2. The output signals from the Hall generators H1 and H2 are of approximately trapezoidal waveform, and vary in proportion to the magnetic flux distribution of the field magnet 1 of the magnetorotor, as the magnetorotor rotates. The output voltages from the Hall generators H1 and H2 are respectively separated by an electrical angle of 90.degree., i.e. differ in phase by 90.degree..
The bases of transistors X3, X4 and X5, X6, which constitute four differential switching amplifiers, are respectively connected to the voltage terminals of the Hall generators H1 and H2, whereby The collector currents of the transistors X3, X4, X5 and X6 are sequentially switched in accordance with the output voltages from the Hall generators H1 and H2. As a result, the transistors X7, X8, X9 and X10, having the bases thereof respectively connected to the collectors of transistors X3, X4, X5 and X6, are sequentially switched on and off. In this way, switching control is implemented of the motor drive currents (stator currents), designated as I1 to I4, which flow through the armature windings L1, L2, L3 and L4 respectively, these windings being respectively connected to the collectors of transistors X7, X8, X9 and X10. Rotational torque is thereby generated by the motor.
Transistor X2 supplies current to the emitters of the four differential switching amplifier transistors X3 to X6, with the level of this current being determined by resistors R2 and R3. A transistor X1 serves to detect the difference between the torque control voltage Vt applied to an input terminal 2 and a current sensing voltage Vi which is developed across the current sensing resistor R1 which constitutes the voltage sensor R. The current sensing voltage Vi thereby produced by R1 is proportional to the sum I of the currents I1, I2, I3 and I4 within the armature windings L1 to L4 respectively, i.e. I=I1+I2+I3+I4.
The collector current of transistors X2 is thus made to vary in proportion to the voltage difference (Vt-Vi) that is thus derived by the adder A, i.e. the total current which flows through the four differential switching amplifiers is made proportional to that voltage difference.
The portion of FIG. 1 comprising the blocks A, G, and R is connected as a closed-loop negative feedback system. Assuming that the gain of the portion of the circuit of FIG. 2 that corresponds to block G in FIG. 1 is sufficiently high, then the loop gain of this feedback system is given as I=Vt/R1. In this way, the motor drive current is made proportional to the torque command voltage Vt.
A current feedback type of brushless DC motor of the form described above with reference to FIGS. 1 and 2 has the advantage that the motor characteristics are relatively unaffected by the effects of long-term drift or ambient temperature variations upon the characteristics of the Hall generators or the drive transistors. This is a significant advantage over brushless DC motors of the type in which the output voltage from the Hall generators is directly amplified to be converted into armature winding current.
With a brushless DC motor of the form shown in FIG. 3, the magnetorotor is made up of a vertically oriented rotor shaft 3, which is supported to be freely rotatable on a bearing 8, a yoke 4 which is fixedly attached to the rotor shaft 3, a field magnet 1 (annular magnet 1) which is fixedly attached to the yoke 4, a magnet 5 fixedly attached to the yoke 4, for generating a magnetic field in a speed sensing signal generator. The motor further consists of a stator made up of a base 6, armature windings L1 to L4 which are fixedly mounted on the base 6, a baseplate 7, position-sensing Hall elements H1 and H2, etc.
With a prior art current feedback type of brushless DC motor such as that described above, the product of the effective magnetic flux produced by field magnet 1 of the magnetorotor which is magnetically linked with the armature windings L1 to L4 and the current which flows in the armature windings L1 to L4 is constant with respect to time. However torque ripple is produced as the motor rotates. The mechanism whereby torque ripple is generated will now be described, for the prior art 6-pole brushless DC motor discussed above. FIG. 5 shows the variation with time of the average amount of magnetic flux linkage between the magnetic flux produced by the magnetorotor and the respective torque generating portions L.sub.T the armature windings L1 to L4, under the condition that the magnetorotor is rotating with a constant value of torque control voltage applied. The respective values of linkage flux for each of the pairs of torque-generating portions L.sub.T of each of the four armature windings L1 to L4 are respectively denoted as .phi.1, .phi.1, .phi.3 and .phi.4. The linkage magnetic flux is plotted along the vertical axis and the angle of rotation of the magnetorotor (expressed as an electrical angle) is plotted along the horizontal axis. The values plotted along the vertical axis have been normalized to a maximum value of 1, and this has also been done for the relationships shown in FIGS. 6 to 8 described hereinafter.
As shown, the variation with time of the average amount of linkage flux between the field magnet 1 of the magnetorotor and the torque generating portions L.sub.T of the armature windings L1 to L4, has a generally sinusoidal waveform. This waveform is determined, broadly speaking, by the shape of the armature windings L1 to L4 and by the angular width of torque-generating portion L.sub.T of each armature winding, as measured about the axis of rotation.
The magnetization pattern of the field magnet 1 is close to magnetic saturation, other than in portions of that pattern which are close to the boundaries between the magnetic poles. The magnetic flux close to the central region of each pole, is oriented vertically at the surface of the field magnet 1 (i.e. extending generally along the axis of rotation), so that the pattern of magnetic flux distribution is approximately trapezoidal. However at portions of the magnetic flux which are displaced from the surface of the field magnet, the orientation of the magnetic flux at positions close to the boundaries between magnetic poles is not vertical with respect to the surface of the field magnet 1. As shown in FIG. 3, the armature windings L1 to L4 are disposed with a fixed separation from the surface of the field magnet 1 of the magnetorotor. As a result, the magnetic flux in the vicinity of the boundaries between the magnetic poles of the field magnet 1 will not be vertically linked with the armature windings L1 to L4. It is for this reason that a substantially sinusoidal variation with time occurs for the average amount of linkage flux between the magnetic flux of the field magnet 1 and the torque generating portions of the armature windings L1 to L4. In addition, the circumferential width of each of the torque generating portions of the armature windings L1 to L4 is finite, so that the amount of linkage flux of the magnetic flux of the field magnet 1 and the armature windings is, to some extent, averaged. As a result, the variation with time of the amount of linkage flux between the magnetic flux of the field magnet 1 and the torque generating portions of the armature windings L1 to L4 will have the type of sinusoidal form shown in FIG. 5.
In FIG. 5, for ease of understanding the process of torque generation, the magnetic flux polarity is indicated as a positive quantity, and only the magnetic flux polarity that results in torque generation is shown. The occurrence of linkage flux of the opposite polarity, which would be indicated as a negative quantity and does not result in torque generation, is omitted from the diagram.
FIG. 6 is a diagram to illustrate the variation with time of the average amount of flux linkage between the magnetic flux of field magnet 1 of the magnetorotor and the torque generating portions of armature windings L1 to L4, but with this variation being drawn as the variation with time of the currents I1 to I4 which respectively flow in the armature windings L1 to L4. The conduction angles and waveforms of these current flows are determined by the output voltages from the position sensing Hall generators H1 and H2 and by the operating conditions of the four differential switching amplifiers.
FIG. 7 shows the variation with time of the effective torque generated by the armature windings L1 to L4, in correspondence with the magnetic flux variations .phi.1 to .phi.4 shown in FIG. 5, and the variations in currents I1 to I4 which pass through the armature windings L1 to L4 shown in FIG. 6. In FIG. 7, the variation with time of the torques T1 to T4 which are respectively generated by armature windings L1 to L4 are shown, i.e. the torque waveforms. The above values of torque, magnetic flux and current are related as follows:
torque T1=.phi.1.times.I1 PA0 torque T2=.phi.2.times.I2 PA0 torque T3=.phi.3.times.I3 PA0 torque T4=.phi.4.times.I4
FIG. 8 shows the combined torque that is produced by the torques T1 to T4 which are generated by the armature windings L1 to L4 and are shown in FIG. 7. The level of torque ripple is produced, equal to approximately 30% of the peak value of torque, with the prior art type of 2-phase 4-input brushless DC motor described above with reference to FIGS. 1 and 2.
As can be understood from FIG. 8, since the amount of torque ripple is proportional to the stator current I, the absolute value of torque ripple will increase as the motor load increases, so that jitter, wow and flutter will be produced as the motor rotates, with the level of these being in proportion to the motor speed of rotation. These factors have a serious adverse effect upon the performance of the motor.
In order to solve the disadvantages of such prior art types of control circuit for a brushless DC motor, the present applicant has disclosed a configuration and a method of operation for a brushless DC motor, in U.S. Pat. No. 4,455,514. With the motor control circuit described in that patent, additional Hall elements are provided for producing sensing signals which are utilized to detect the level of motor torque, with these Hall generators being positioned in correspondence with the position-sensing Hall generators having the function described hereinabove. The torque sensing Hall generators are subjected to the magnetic flux of the field magnet, and in addition are supplied with currents which vary in accordance with the currents through the armature windings, for thereby generating a torque sensing signal. This torque sensing signal is employed in a negative-feedback control loop, to control the armature winding current, and hence hold the level of torque produced by the motor constant at a required value. This prior art invention fully attains the desired objectives. However since it is necessary to use a number of Hall generators for torque sensing that is equal to the number of Hall generators used for position sensing, such a brushless DC motor control circuit has the disadvantage that the configuration is complex. Improvements in this respect are therefore desirable.